Alternative splicing increases the coding potential of human genome by producing multiple proteins from a single gene. It is also associated with a growing number of human diseases.
SMA is an often-fatal genetic disorder resulting from the loss of the SMN protein encoded by the Survival Motor Neuron SMN gene. The SMN genes, SMN1 and SMN2, are located on chromosome 5 and SMA is caused by the loss of SMN1 from both chromosomes. SMN2, while being almost identical to SMN1, is less effective at making the SMN protein. The severity of SMA is affected by the efficiency at which SMN2, of which there are several copies, produces the SMN protein.
SMN1 encodes a ubiquitously expressed 38 kDa SMN protein that is necessary for snRNP assembly, an essential process for cell survival. A nearly identical copy of the gene, SMN2, fails to compensate for the loss of SMN1 because of exon 7 skipping, producing an unstable truncated protein, SMNΔ7. SMN1 and SMN2 differ by a critical C to T substitution at position 6 of exon 7 (C6U in transcript of SMN2). C6U does not change the coding sequence, but is sufficient to cause exon 7 skipping in SMN2.
Current treatment for SMA consists of prevention and management of the secondary effect of chronic motor unit loss. Currently, there are no drug therapies available for the treatment or prevention of SMA.
Antisense technology, used mostly for RNA downregulation, recently has been adapted to alter the splicing process. Effective agents that can alter splicing of SMN2 pre-mRNAs are likely to be useful therapeutically.